1. Field of the Invention
The present invention relates to an air current classifier capable of producing a high-velocity whirling stream on a powder material fed into a classifying chamber, to centrifugally separate the powder material into fine powder and coarse powder. It further relates to an apparatus for preparing a fine powder, equipped with said air current classifier and a jet mill, a process for preparing a toner, having a classification step using said air current classifier, and an apparatus for preparing a toner, having said air current classifier as a classifying means.
2. Related Background Art
As air current classifiers, Classiclon (Nagoya Industrial Science and Technology Laboratory Reports 8 [4] 235,1959), Iitani's classifier (The Journal of the J.S.M.E. Society 59 [3] 215, 1956), etc. have been hitherto proposed. According to these, the size of the particles to be separated depends on the shape of the machine, and it is difficult to control the cut size. These classifiers employ a system in which powder materials are fed into a classifying chamber from one place, and have the problems that the powder can be poorly dispersed, and can be classified with a very low accuracy if the materials are fed at an accelerated velocity, resulting in a shift of the size of separated particles to the coarse side. As a means for solving these problems, Japanese Patent Laid-Open No. 54-48378 proposes a method that enables control of the height of a classifying chamber, and Japanese Patent Laid-Open No. 54- 79870 proposes a method in which a guide cylinder in the shape of a cyclone is mounted on a classifying chamber. Those comprising a combination of these are actually put into practical use.
FIG. 5 schematically illustrates a classifier having been put into practical use.
However, in the air current classifier of this type (comprising the combination of the techniques disclosed in Japanese Patent Laid-Open No. 54- 79870 and No. 54-48378) as illustrated in FIG. 5, a powder material feeding portion to the classifying chamber is in the shape of a cyclone, where a guide cylinder 50 is upright provided at the upper central part of an upper cover 60, and a feed cylinder 80 is connected to the upper peripheral surface of the guide cylinder 50. The feed cylinder 80 is so connected that the powder material fed to the periphery of the guide cylinder 50 through the feed cylinder 80 may be led in the direction tangent to the inner circumference of the guide cylinder. The powder material may be fed from the feed cylinder 80 to the guide cylinder 50, so that the powder material falls down while whirling along the inner circumference of the guide cylinder 50. In this occasion, the powder material falls down in belt-like fashion along the inner circumference of the guide cylinder 50 from the feed cylinder 80, and hence the powder material is flowed into a classifying chamber 40 in a non-uniform dispersion and density (i.e., the powder material is flowed into the classifying chamber from only part of the inner circumference of the guide cylinder), resulting in poor dispersion. If the throughput is made greater, there may arise the problem that the aggregation of powder material becomes more liable to occur, making it impossible for the powder material to be further dispersed and also making is impossible to carry out classification in a high accuracy.
A large quantity of the air that carries the powder material results in a large quantity of the air flowed into the classifying chamber, and hence there arises the problem that the velocity of the particles whirling toward the center in the classifying chamber becomes greater to make larger the size of separated particles. Accordingly, in an attempt to make small the size of separated particles, the air is usually let out from an upper part 140 of the guide cylinder. However, a large quantity of the air let out may sometimes bring about a practical problem that part of the powder material also is released therefrom and lost.
Japanese Utility Model Laid-Open No. 54-81172 proposes an air current classifier comprising, as illustrated in FIG. 6 and FIG. 7 (a cross section along the line II--II), a spiral feed cylinder 150 provided at the peripheral part of a surrounding wall of a classifying chamber in such a manner that the passing area may be gradually reduced as it reaches from the starting end area at the inlet side to the terminal area, a number of louvers provided at a circular communicating area provided between this feed cylinder and the classifying chamber, a circular high-pressure air feeding chamber further provided around the periphery of said feed cylinder, and a plurality of nozzle holes 220 formed in the circumferential direction of the inner peripheral wall of said feed chamber and opened in the same direction as said louvers. In this air current classifier, an improvement has been made so that the powder material fed and dispersed at a uniform velocity from the openings between the louvers may be flowed into the classifying chamber. Since, however, the high-pressure air (A) is so designed as to be jetted from the nozzle holes 220, there is the problem that turbulances are caused by the high-pressure air to lower the accuracy of classification.
Now, some may contemplate a feeding method in which the high-pressure air has been omitted as illustrated in FIG. 8 and FIG. 9 (a cross section along the line III--III). In this method, however, the powder material is flowed along the inner wall of the periphery of the feed cylinder 150 by the action of a centrifugal force, so that it is not uniformly flowed into the classifying chamber from the louvers and is flowed thereinto in a large quantity from the terminal area, and hence it is difficult even to obtain the effect obtainable in the apparatus illustrated in FIGS. 6 and 7.
Moreover, since in the apparatus illustrated in FIGS. 6 and 7 the whirling stream that contributes to the classification in the classifying chamber is only the air flowed in from the openings between the louvers 70, the powder material moves along the periphery of the classifying chamber in the same way as the effect of a cyclone, by the action of a centrifugal force produced by the whirling air current flowed in from the openings between the louvers 70, so that there may arise the problem that the powder material more strongly tends to be captured to make fine powder liable to be included in the coarse powder side.